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Biomedical subjects

Barbara A Fink

Publications and source records attributed to Barbara A Fink.

11 recordsLinked to original sources
PubMed2006

Factors influencing the measurement of oxygen shortfall of the human cornea: sequencing of test conditions.

PURPOSE: The effects of sequencing of test conditions, in this case contact lens thicknesses, on the measurement of the oxygen shortfall of human corneas were studied. METHODS: Corneal oxygen uptake rates were measured with a Clark-type polarographic electrode on the central, unanesthetized right corneas of 14 human subjects. Measurements were made under the following conditions: (1) the normal open eye; (2) after 5 min of static (without blinking) wear of each of seven rigid gas permeable lenses of seven center thicknesses (0.18, 0.12, 0.16, 0.20, 0.24, 0.28, and 0.32 mm); (3) after 5 min of static wear of a polymethylmethacrylate (PMMA) contact lens. Lens thicknesses were randomly assigned numbers, which were sequenced in seven cycles. Two subjects were assigned to each sequencing cycle, and each subject participated in two identical sessions. RESULTS: The interaction of order x thickness was determined to be insignificant (F=0.99; p=0.5101). The effect of lens order was also insignificant (F=0.76; p=0.6239), indicating that the order of lens placement did not affect the measured corneal oxygen shortfall. Not surprisingly, the analysis indicated a significant effect of lens thickness on corneal oxygen shortfall (F=3.94; p=0.0032). CONCLUSIONS: The sequencing of lenses of various thicknesses on the cornea does not affect the measurement of corneal oxygen shortfall.

Adult↗
PubMed2006

Baseline factors predictive of incident penetrating keratoplasty in keratoconus.

PURPOSE: To identify baseline demographic and clinical factors associated with undergoing penetrating keratoplasty (PK) in a prospective cohort of 1,065 keratoconus patients followed for eight years in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study. DESIGN: Multicenter, prospective, observational cohort study. METHODS: We report the rate of PK over eight years and baseline factors predictive of PK in 1,065 patients who, at the time of study enrollment, had not undergone PK in either eye. RESULTS: Eighty-two percent of patients completed the eight-year close-out visit. Twelve percent (126 of 1,065) had PK in one (9.3%) or both eyes (2.5%). Baseline factors associated with increased likelihood of PK included younger age, steeper keratometric values, worse visual acuity, corneal scarring, poorer contact lens comfort, and poorer vision-related quality of life. The percent of eyes undergoing PK was 15% for patients 40 years old and younger, 28% for eyes with a steep keratometric value greater than 52 diopters, 33% for visual acuity less than 20/40, and 24% for eyes with corneal scarring. CONCLUSIONS: The CLEK Study confirmed previous reports of the increased likelihood of PK associated with corneal scarring, steeper keratometry values, poorer visual acuity, and poorer contact lens comfort. The CLEK Study is among the first to report an increased risk of PK associated with younger age, worse vision-related quality of life, and flatter contact lens fits. Knowledge of these factors is beneficial to clinicians in patient education and may be useful in disease management.

Adolescent↗
PubMed2006

Rigid gas-permeable contact lens base curve radius and transmissibility effects on corneal oxygen uptake.

PURPOSE: The purpose of this study is to determine the effects of rigid gas-permeable contact lens thickness, base curve radius, and material permeability on corneal hypoxic stress. METHODS: Corneal oxygen uptake rates were measured with a Clark-type polarographic electrode on the right eye of 10 human subjects for the normal open eye (air) and after 5 minutes of static wear of rigid contact lenses of four cornea-to-contact lens base curve fitting relationships: 0.2 mm steeper-than-K (STK), 0.1 mm STK, on K, and 0.1 mm flatter-than-K (FTK). There were also four materials (polymethylmethacrylate [Dk=0], lotifocon B [OP-2, Dk=15.9], lotifocon A [OP-3, Dk=30], lotifocon C [OP-6; Dk=60]) and three center thicknesses (0.14, 0.28, and 0.53 mm for the OP-6 lenses and 0.14 mm for all other materials) with all other parameters being constant. Each subject participated in two identical sessions. A repeated-measures analysis of variance was performed to compare the mean response across lens materials/thicknesses and the four curvature values. RESULTS: Significant differences were found only for lens material/thickness (p<0.0001). Although OP-2 and OP-6 (0.53 mm) were manufactured to have the same Dk/t, post hoc comparisons showed that the oxygen uptake rate with OP-6 (0.53 mm) was significantly lower. The same is also true for OP-3 and OP-6 (0.28 mm), with OP-6 (0.28 mm) having a significantly lower oxygen uptake rate. CONCLUSIONS: In contrast to past studies with PMMA, cornea-to-contact lens base curve fitting relationship, with the lens materials and designs studied here, did not affect corneal hypoxic stress, and thick, high Dk lenses resulted in less change in corneal response than did thin, lower Dk lenses of the same Dk/t. This is attributed to the lens reservoir effect that has been previously described.

Adult↗
PubMed2006

Interferometric imaging of the full thickness of the precorneal tear film.

A method is described for recording interference images from the full thickness of the precorneal tear film (PCTF). Simultaneous images are recorded by two video cameras. One camera responds to broadband spectral illumination and records interference from the superficial lipid layer of the tear film. The other camera uses narrowband illumination and records interference from both the lipid layer and the full thickness of the PCTF. Thus the full-thickness interference fringes are derived from the difference between, or ratio of, narrowband broadband images. This method has the potential for evaluating the role of both tear film flow and evaporation in tear film thinning and breakup. It therefore may be applied to the analysis of dry eye disease.

Adult↗
PubMed2005

Differences in keratoconus as a function of gender.

PURPOSE: To characterize gender differences in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study. DESIGN: Observational, longitudinal study. METHODS: A total of 1209 subjects at 16 clinics. For eye-specific categorical variables, the number of eyes per subject with the characteristic was counted. For eye-specific continuous variables, the mean of both eyes was calculated. Multivariate linear (for continuous outcomes) and logistic (for categorical outcomes) regression models were created for each baseline characteristic with statistically significant (P < or = .05) differences between men and women. Age, race, education, and corneal curvature were covariates. RESULTS: The women were older, more likely to report a family history of keratoconus, more likely to be nonwhite, and less likely to complete college than men. Vogt's striae and monocular and binocular high-contrast entrance acuity were the only visual characteristics that varied between men and women in the multivariate model. Women were more likely than men to report ocular symptoms of dryness and complaints based upon a composite score of ocular symptoms. Women reported more hours per day of near work and were less likely to report the ability to wear contact lenses for enough hours to permit reading at home in the evening. Women reported more visits to their eye care practitioner during the previous 12 months. NEI-VFQ results revealed differences in self-reported difficulty with distance activities and driving. CONCLUSIONS: Gender differences exist in patient history, vision, and ocular symptoms in keratoconus patients.

Adult↗
PubMed2005

The effects of high-Dk rigid contact lens center thickness, material permeability, and blinking on the oxygen uptake of the human cornea.

PURPOSE: The human corneal oxygen uptake responses associated with the static (nonblinking) and dynamic (blinking) wear of five rigid gas-permeable materials with high oxygen permeabilities were determined for three different center thicknesses and compared with the responses for the normal open eye and severe hypoxic stress (static wear of polymethylmethacrylate). METHODS: Corneal oxygen uptake rates were measured with a Clark-type polarographic electrode during two sessions with each of 10 human subjects. Measurements were made on the right eye for the normal open eye (air) and after 5 minutes of static and dynamic wear of polymethylmethacrylate and five rigid gas-permeable contact lens materials: Fluoroperm 92 (paflufocon A, Dk = 92), Fluoroperm 151 (paflufocon D, Dk = 151), 1992 Menicon SF-P (melafocon A, Dk = 102), 1995 Menicon SF-P (melafocon A, Dk = 159), and Menicon Z (tisilfocon A, Dk = 163-250). Lenses were manufactured in three different center thicknesses (0.12, 0.16, and 0.20 mm), with all other parameters remaining constant. Repeated-measures analysis of variance was used and included lens material (five levels), blinking condition (two levels), and lens thickness (three levels) as within-subject effects. RESULTS: Significant differences were found in corneal oxygen responses to lens material (p < 0.001) and lens thickness (p < 0.001), with lenses of lower oxygen permeability and thicker lenses being associated with higher oxygen uptake. No statistically significant differences were noted between static or dynamic wear of the lens materials (p = 0.59). CONCLUSIONS: For those very high Dk rigid lens materials studied here, moderate changes in lens thickness or material permeability may result in modest differences in corneal hypoxic relief, whereas blinking results in no significant improvement to corneal oxygenation.

Adult↗
PubMed2003

Tear exchange and oxygen reservoir effects in silicone hydrogel systems.

PURPOSE: The effects of oxygen reservoir and tear exchange are known for conventional hydrogel contact lenses. This study attempted to (1) confirm their presence in lenses of a silicone hydrogel (SH) material and (2) evaluate their individual and combined contributions to hypoxic relief of the cornea. METHODS: Corneal oxygen uptake rates were measured polarographically for 10 OD corneas immediately after 300-sec periods (equivalent to 60+ blink cycles) of: (1) nonblink wear of a 0 Dk/L polymethylmethacrylate (PMMA) cap lens; (2) nonblink wear of that cap lens with a SH lens inserted between it and the cornea; (3) regular blinked wear of that same lens combination; (4) nonblinked wear of the SH lens alone; and (5) normal open-eye, non-lens wear. The rates for each eye had a ratio with its own baseline (no lens) rate, and the total sample mean was calculated for each condition from those individual eye means. RESULTS: (1) Under nonblink conditions, a 28% reduction in corneal oxygen demand was observed when a SH lens was inserted under the PMMA cap lens versus without. (2) When regular blinking was added to that lens combination, corneal oxygen demand decreased another 8%, for a total of 36%. The combination of SH insertion and blinking did achieve a statistically significant difference (P<0.05 by the Dunnett test) from the nonblink, 0 Dk/L, maximum deprivation condition. CONCLUSION: The statistically significant hypoxia reduction observed with the SH lens insertion and blinking indicates the additive presence of two factors: (1) a lens reservoir effect caused by the SH lens and (2) a bulk-flow tear exchange effect caused by blinking. Their respective contributions to the reduction of corneal oxygen demand over the period studied were found to be in the ratio of 3.4:1.

Adult↗
PubMed2002

Between-eye asymmetry in keratoconus.

PURPOSE: To report baseline differences between eyes on key variables in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study cohort compared with a retrospectively assembled group of myopic contact lens wearers without ocular disease. METHODS: A total of 1,079 keratoconus patients who had not undergone a penetrating keratoplasty in either eye before their baseline visit were enrolled and examined at baseline. Records from 330 contact lens-wearing myopes were reviewed. Corneal curvature (keratometry), visual acuity, refractive error (manifest refraction), and corneal scarring were measured. RESULTS: The mean differences between keratoconic eyes are as follows (better eye-worse eye for each variable, separately). Flat keratometry: -3.59 +/-4.46 D and steep keratometry: -4.35 +/-4.41 D; high-contrast best-corrected visual acuity: 7.30 +/-6.83 letters; low-contrast best-corrected visual acuity: 8.53 +/-7.51 letters; high-contrast entrance visual acuity: 9.03 +/-8.40 letters; low-contrast entrance visual acuity: 9.43 +/-7.88 letters; spherical equivalent refractive error: 3.15 +/-3.84 D; and refractive cylinder power 1.55 +/-1.42 D. Twenty-one percent of the keratoconus patients had corneal scarring in only one eye. There is an association between patient-reported unilateral eye rubbing and greater asymmetry in corneal curvature, and between a history of unilateral eye trauma and greater asymmetry in corneal curvature and refractive error, with the rubbed/traumatized eye being the steeper eye most of the time. CONCLUSIONS: Keratoconus is asymmetric in the CLEK Study sample.

Adult↗
PubMed2002

Enhancing RGP contact lens performance: comparing back surface options.

BACKGROUND: In some cases, rigid gas-permeable (RGP) contact lenses may be the best--or only--means of refractive correction. High Dk RGP materials have markedly reduced hypoxia under those lenses. With aspheric lens back surface designs, post-lens circulation may be enhanced as well by maximizing the provision of nutrients and the clearance of metabolic by-products, toxins, and debris, while minimizing the risk of lens binding. METHODS: Performances of four aspheric back surface RGP designs were compared with a spherical optic zone and peripheral curve back surface reference design. Oxygen uptake rates were measured for each of 40 wearing combinations (five lens designs fitted to each of eight eyes) under non-blink and blink conditions for their effectiveness in reduction of post-lens hypoxia. Hypoxic reductions with blinking were scaled in exchange efficiency (EE) units, and used as relative indicators of post-lens tear exchange. RESULTS: Combined exchange efficiency index (EE) scores (averaged responses across all eight eyes) for each of the five lens back surface designs ranged from a best overall performance of +13.9 EE units down to only +2.5 EE units, with the spherical reference design averaging +8.9 EE units. Tear pump efficiency of each of the eight eyes (averaged responses across all five lens designs) ranged from a high of +12.3 EE units down to -9.8 EE units (i.e., worse than the non-blink condition of 0 EE units). Among the 40 eye-lens back surface design combinations studied here, the highest exchange efficiency score registered was +28.4 EE units, the lowest being -13.8 EE units. CONCLUSIONS: Aspheric lens back surface and/or peripheral curve designs were found to vary significantly in their post-lens exchange efficiency performances, but no "universal problem-solver" design was found among the five we investigated.

Adult↗
PubMedDate not supplied

The thickness of the tear film.

Measurements of the thickness of the pre-corneal tear film, pre-lens tear film, post-lens tear film, and the lipid layer on the surface of the tear film are summarized. Spatial and temporal variations in tear film thickness are described. Theoretical predictions of tear film thickness are discussed. Mechanisms involved in the upward drift of the tear film after a blink, and in the formation of dry spots, are considered.

Animals↗